Internally routing or Hiding Di2

yes. maybe a bayonet fork or front fairing is a better idea.

Front brake hose in front of headtube under fairing.
rear stuff on top of stem (under a cover) into a top tube that is slightly higher

Inspiration:

(just the shapes , not the actual brake setup)

or extend the headtube (with a extra tube :)) like this:

https://cyclingtips.com/2019/12/bikes-of-the-bunch-stinner-aero-camino/

just ehhmm... neater than these examples :)

I'm assuming Columbus or Reynolds will produce them eventually. If not, it's time to get some 4130 on the lathe, and see if Peter at Ceeway sells the braze-in lower Columbus integrated bearing races separately from the top ones.